An optical communication system, such as a free space optical communication system, can use modulated optical signals to convey information. The optical signal can be generated by an optical source having a relatively narrow spectral bandwidth, such as a laser.
A laser optical source can be optically modulated using any one of a variety of methods. For example, the laser output can be optically modulated internal to the laser by modulating a current to the laser. The current can affect the light intensity of the laser output according to a characteristic light to current curve. An alternative to internally modulating the bias current of the laser is to utilize an external optical modulator positioned in the output optical signal path of the laser.
A quantum well optical modulator that utilizes electro-absorption to modulate the optical signal can be configured to provide relatively high modulation index while simultaneously supporting high modulation bandwidths. A drawback to such modulators is that their modulation characteristics are quite temperature dependent. The temperature effects can be compensated by the application of a DC bias voltage in addition to the AC modulation voltage.
In optical communication systems that are specified to operate over a relatively large temperature range, the magnitude of a reverse bias voltage applied to the modulator may change drastically. For example, to operate an electro-absorption optical modulator over a temperature range of −40° C. to +70° C. may require a reverse bias voltage that ranges from 40 volts to greater than 90 volts.
It is extremely difficult to manufacture a quantum well optical modulator having a high breakdown voltage. In particular, it is difficult to manufacture an electro-absorption optical modulator that is able to withstand an 90 volt reverse voltage bias at a high manufacturing yield.
Increasing the yield of high breakdown voltage optical modulators would likely entail complex Metal-Organic Chemical Vapor Deposition (MOCVD) growth processes and epitaxial wafer design controls. The technology and development time and costs required to produce a high breakdown voltage optical modulator makes it unsuitable for application in optical communication systems that require immediate low cost solutions.